The present invention relates to a new and improved method of, and apparatus for, monitoring the run-up to speed of--preferably explosion protected--asynchronous motors by detecting a physical magnitude which is dependent upon the motor rotational speed within a permissible motor blocking or cutoff time.
During the monitoring or supervising of the run-up to speed of motors it is absolutely compulsory for certain applications, such as for instance for motors protected against explosions where there is prevented too pronounced heating of the windings through which the current flows by blocking or shutting down the motor upon improper speed run-up, that the permissible motor blocking or cutoff time t.sub.E to chosen smaller than the startup time t.sub.A. With prior art methods for monitoring the run-up to speed of asynchronous motors it is conventional practice to detect the motor rotational speed directly by means of a rotational speed monitor, after turning-on the motor, and following expiration of a predetermined time-delay falling within the motor blocking time. If after having applied voltage to the motor the latter, following expiration of the set time-delay, still has not reached a predetermined rotational speed, e.g. 40% n, then such motor is again disconnected from its power supply or voltage source. Yet, such monitoring method is expensive and complicated since it is necessary to mount a device for monitoring the rotational speed of the motor upon its shaft, and furthermore, the reproducibility of the obtained measurement result is questionable.